1. Field of the Invention
The present invention relates to a thin-film transistor which is formed on an insulating substrate.
2. Description of the Related Art
As a display device of, e.g. OA equipment, which displays image information and character information, there is known an active-matrix flat-panel display which uses thin-film transistors (TFTs). In recent years, with the development of multi-media communication technologies, attention has been paid to an integrated-function type display as a next-generation display, which is called a “system-on-panel” display. This integrated-function type display is designed for personal use, has a small size and light weight, and has a high resolution and a high image quality. Moreover, peripheral functions relating to image display, such as driver circuits, memory circuits, DA converters and image processing circuits, are integrated on a display panel.
Conventionally, MOS (Metal Oxide Semiconductor) transistors, which fall in a category of field-effect transistors, are mainly used as TFTs which are formed on a display glass substrate. The MOS transistors are widely used since the MOS transistors can advantageously constitute digital circuits such as shift registers and display pixel switches of displays.
In recent years, attempts have been made to increase the added values of display devices by integrating various functions, aside from a display function, on the display substrate. Jpn. Pat. Appln. KOKAI Publication No. 2005-18088, for instance, discloses a liquid crystal display device having an input function using light from, e.g. a light pen, by providing photoelectric conversion elements in individual pixels.
In the case of the above-described example, a function of detecting and amplifying a photoelectric current is needed. However, in a source-grounded type amplifying circuit which is composed of MOS transistors, an input impedance is high and the photoelectric current cannot directly be amplified. In such a case, the photoelectric current is received by providing a current buffer circuit having a gate-grounded structure. In order to obtain a sufficient gain with the MOS transistor alone, however, the circuit structure becomes complex. Thus, there is the problem that a sufficient detection sensitivity is hardly obtained with a simple-structured circuit.
On the other hand, a bipolar transistor is known as a transistor in which a current is input as an input signal. In the case of using a single-crystal Si or SOI (Silicon On Insulator) substrate, the BiCMOS (bipolar CMOS) technology has already been established in which bipolar transistors and CMOS transistors are provided on the same substrate in a hybrid fashion, and these transistors are selectively used, as needed. In conventional techniques, however, it is difficult to implement such a structure that two kind of devices are provided in a hybrid fashion on a low-heat-resistant substrate such as a glass substrate. The inventor of the present invention studies techniques for meeting this demand, and has found that the reason for this is that a minimum processing dimension, which is realized on a large-sized glass substrate of, e.g. about 1 m, is about 3 microns at most. Moreover, another reason appears to be that the crystal quality of a Si thin film, which can be formed on a glass substrate at low temperatures, is poor, and a minor carrier lifetime in the Si thin film layer is too short to invoke the bipolar action.